1. Field of the Invention
This disclosure relates to a system and method for treating bone fractures, bony ingrowth and osteoporosis, and in particular, to a dynamic system and method for applying high frequency, low stress stimulation to a fracture site for accelerating the healing process and improving bone strength.
2. Description of Related Art
Weakened bone structure and improperly healed or slowly healing bone fractures may result in reduced quality of life. Quality of life may be improved for patients with bone fractures by ensuring rapid healing and by inhibiting the loss of bone mineral content (bone mass), and therefore bone strength, associated with fractures. Metabolic bone diseases, such as osteoporosis, also reduce the quality of life.
Osteoporosis is a pernicious disorder usually, but not exclusively, afflicting elderly women. The osteoporotic state can also be manifested by those who are confined to bed and even to astronauts who are subjected to prolonged weightlessness. Osteoporosis occurs through a decrease in bone mass which makes the afflicted bones more fragile and more susceptible to breaking.
The reduction in bone mass from osteoporosis results when destruction outpaces bone formation. The balance between destruction and formation is affected by hormones, calcium intake, vitamin D and its metabolites, weight, smoking, alcohol consumption, age, genetic determinants and especially exercise or other forms of dynamically loading the bone tissue as well as many other factors. Considering the vast array of factors which can compromise the healing process, any form of stimulation that can accelerate, augment and/or ensure the healing process are greatly needed.
Osteoporosis is not easily determined in its early phases as physical deformity is not yet evident. Because osteoporosis develops progressively, early diagnosis and appropriate treatment may avoid a serious condition. Appropriate diet and exercise can be used in early years to prevent the damaging effects of osteoporosis later in life. Methods for maintaining or promoting bone growth are described in numerous patents. For example, McLeod and Rubin, U.S. Pat. Nos. 5,103,806, 5,191,880, 5,273,028 and 5,376,065 collectively describe means and methods for promoting bone growth and preventing bone loss. The method described in the above referenced patents relates to a mechanical vibrational loading of bones to promote growth in a non-invasive procedure. McLeod and Rubin, U.S. Pat. Nos. 5,103,806, 5,191,880, 5,273,028 and 5,376,065 are all incorporated herein by reference. In addition, U.S. Pat. No. 5,046,484 to Basset et al. describes impact loading a patient's bones by dropping the patient from a predetermined drop excursion.
Mechanical loading on bone tissue at strains of between about 0.5 to about 500 microstrain and induced within a predetermined frequency range can prevent bone loss and enhance new bone formation. Such mechanical bone loading of tissue may be introduced by various apparatus, including vibrating floor plates and chairs, electrical stimulation of muscles, isometric exercises, modulated ultrasound or transducers attached to the skin or external fixation devices to focus energy to the fracture site.
As with osteoporosis, bone fracture may result in weakened bone structure. Improperly healed fractures are often subject to refracture. Delayed or improper union of fractures and failure of skeletal prostheses resulting from inadequate osteogenic responses lead to high morbidity and reduced quality of life. To counter these effects, many techniques have been proposed to increase bone mass and stimulate bone growth. One such technique is described in U.S. Pat. No. 4,993,413 ('413) to McLeod and Rubin which is incorporated herein by reference. The '413 patent describes a method and apparatus for inducing a current and voltage in living tissue to promote, inter alia, bone regrowth. Other techniques include the use of ultrasound waves and the application of mechanical strains directly to the fractured bone to enhance bone regrowth.
Ultrasound delivery systems use pulsed radio-frequency waves (in the MHz range) to treat bone fractures. These systems take advantage of the piezoelectric nature of bones. When ultrasound is applied to the fracture bone, the ultrasound is converted to an electric current in the bone to promote healing by small deflections within the bone. Such systems are described in U.S. Pat. Nos. 5,003,965 and 5,186,162 to Talish et al both incorporated herein by reference. The stresses induced in the bone are of the order of 100 kilopascals (kPa). The ultrasound carrier frequencies are about 1.5 MHz and higher. As the bone deflects in response to the ultrasound, bone growth is promoted. Clinical studies show that exposure of the fracture site for 20 minutes per day to such an ultrasonic stimulus will halve the time necessary to ensure a fully healed fracture.
Studies have been performed to show that cyclical mechanical stimulation has modulated the healing process. One such study is reported in Clinical Orthopedics, Clin. Ortho. & Rel. Res. (1989) 241:36-47 by J. Kenwright and A. E. Goodship (referred to hereinafter as Goodship). In the Goodship study, animals, such as sheep, were used to determine the effects of mechanical stimulation on midshaft tibial breaks of 3 mm. The regimen of treatment included a frequency loading of about 0.5 Hz, representing the walking frequency of the animal. The break or fracture was cyclically loaded for 17 minutes per day to achieve initial displacements between 0.5 mm and 2 mm. Peak stresses were permitted to reach as high as 1.8 megaPascals (MPa). It was noted that a 1 mm initial displacement yielded beneficial results. Initial displacements of greater than 1 mm resulted in an impedance to bone repair. However, Goodship used high displacements (0.5 mm to 1 mm) and stresses which may not be suitable for use in all bone repairs. Also, high displacements and stresses may increase the risk of mechanical failure of the wound healing process, as well as the failure of the fixation device itself.
Since it is desirable to enhance the rate of repair and the strength of the repair in delayed or non-union fractures, a need exists for a system and method for mechanically cycling bones at a high frequency at low levels of displacement. A need also exists for a system and method for improving bone strength in patients with osteoporosis. A further need exists for a system and method for continuing mechanical cycling treatment at different stages in the healing process.